Hi,

I noticed this issue when playing with Better With Mods' HCGloom feature that wouldn't work correctly when stepping outside at night.

Digging into what could be causing this I learned that Minecraft's outside light level never goes below 4. Hardcore Darkness doesn't change this even if it makes everything look pitch black.

As an exercise I made a fork of your repository, updated it to Minecraft 1.12.2, got my hands dirty and fixed the issue. You can see the changes in my commit here:

commit b49ddd9285e517a712a6f9799370bb134d88d72d

Note:
The added change to getSunBrightnessBody makes it so the detected light level corresponds to the displayed one. In other words, as soon as the light level reaches 0, the world goes completely black and the gloom starts. It effectively syncs up getSunBrightnessBody and getSunBrightnessFactor.

I'm hoping you'll consider taking a look at this and adding it to your mod.

I would also love to see this happen, mostly for compatibility with the newest version of Grue: Shinoow/Grue#13